Skull Base Anatomy, Variants,
and “Don’t Touch Me” Lesions Nancy J. Fischbein, MD
Neuroradiology Section
Stanford University Medical Center
ASHNR 2017
Session 8: Temporal Bone & Skull Base – Part 2
Monday 9/18/17, 7:45-8:10 a.m. Learning Objectives
• To review the anatomy of the anterior and
central skull base and associated foramina
and soft tissues on CT and MR
• To demonstrate selected anatomic variants
of these regions
• To illustrate some of the common and less
common “don’t touch” lesions of the region
Disclosures
• None
ANATOMY
Anterior Skull Base • Frontal bone
– Squama (vertical
portion)
– Orbital plate
(horizontal portion)
• Floor of anterior
cranial fossa
• Also ethmoid roof
– Frontal sinus
• Ethmoid bone
– Cribriform plate
– Ethmoid sinuses
• Articulation
– Posteriorly w
sphenoid bone,
superiorly w parietal
bone
Adult
Newborn
Orbital plate
Orbital plate
Squama
Squama
Gray’s Anatomy
Ethmoid Bone • Four parts
– Horizontal or
cribriform plate
• Perforated by
foramina for olfactory
nerves
– Perpendicular plate
• Part of nasal septum
– Paired lateral
masses
• Air cell labyrinths
Olfactory recess
Olfactory groove
Crista galli Ethmoid roof
Cribriform
plate
Ethmoid roof (fovea ethmoidalis) is formed by orbital process of frontal bone
Olfactory recess
Ant eth
canal
Post eth
canal
Perpendicular
plate
Crista galli
Olfactory
apparatus
Cribriform level
Gyrus rectus
Kallmann Syndrome
Olfactory recess
Anterior Skull Base: Changes over Time
• At birth, ASB largely cartilaginous
• Ossification begins in roof of ethmoid labyrinth
laterally and spreads to midline
– Crista galli, perpendicular plate of ethmoid begin ~2
months, steady increase to ~14 months (Belden et al, AJNR 1997)
Birth: no ossification 7 mo: thin but complete
bony bridge
3 mo: some ossification;
faint outline of CG
Developmental defects/lesions may affect the anterior skull base. At
all ages, be alert for meningocephaloceles.
Frontonasal cephalocele
31 F found down in apartment,
dx’d w bacterial meningitis.
Frontoethmoidal cephalocele
B frontoorbital cephaloceles
Other developmental lesions may occur 1 day old male w difficulty breathing; also mild hypertelorism
Dx: nasal glioma [no direct intracranial connection identified at surgery; there is also
dysplasia of L anteroinferior frontal lobe and L olfactory apparatus]
These are “don’t touch” in the sense of “don’t touch w/o considering that you may end up in brain and CSF”
• Sphenoid bone
– Body; greater wings; lesser
wings; pterygoid processes
– Multiple skull base foramina and
fissures for vessels and nerves
• Occipital bone
– Basiocciput contributes to clivus
Central Skull Base
Sphenoid bone, posterior surface (Gray s anatomy)
Occipital bone, outer surface (Gray s anatomy)
Central skull base, inner and outer surfaces (Frank Netter, Atlas of Human Anatomy)
Palatine foramina
NP
OC
Pterygoid process
Clivus
Hypoglossal canal
Pterygopalatine fossa
C
Jugular foramen
J J CVJ
PPF VC
FO
FS
C
SPF
FO
FS Carotid canal
C
CT Anatomy of Central Skull Base and Foramina
Ca
Foramen rotundum
Orbit
MCF
IOF
*
Superior orbital fissure Optic canal
SS
FO NP
ST
FR
VC
SS
AC
P
OC
GWS
SOF
IOF
PPF
CS
FR
C PA
SS
FO
NP
Jugular foramen V3 below skull base
MR Anatomy of Central Skull Base and Foramina
Optic
nerve
SOF
PPF
VC
Occipital condyles
Hypoglossal canal
Clivus
Petrous apices
Trigeminal nerves
Chiasm
Sella
Foramen ovale
V3
Optic canals
Ant clinoid
SOF
F. Rotundum
Vidian canal
Central Skull Base: Changes Over Time, CT
16 F
6 F 18 mo F
67 F
10 F
18 mo:
Minimal sphenoid sinus pneumatization
Prominent sphenooccipital synchondrosis
10 yrs:
Larger sinus
Synchondrosis obliterating
Adulthood:
Large fully pneumatized sphenoid sinus
Complete obliteration of sphenooccipital synchondrosis
Central Skull Base: Changes Over Time, MR
• Pneumatization of sphenoid sinus
• Fusion of the spheno-occipital synchondrosis
• Clival marrow: red to yellow
14 mo 3 yrs 8 yrs 52 yrs
Some clival heterogeneity can be normal
Initial MR Correlative CT
50 yr old male
7 yrs later
Same patient, age 57
ANATOMICAL VARIANTS/
“DON’T TOUCH” LESIONS
• Developmental variants or anatomical
aberrations in bone, vessels, soft
tissues
• We don’t want to biopsy them or treat
them with stereotactic radiosurgery
• There are enough to fill entire
textbooks! A few are selected
Notochord • Flexible rod-shaped
body in embryos of
all chordates
– Composed of
mesoderm-derived
cells
– Defines primitive axis
of the embryo
– Located ventral to
neural tube
• In most vertebrates,
persists only as
nucleus pulposus of
intervertebral disc Nguyen R et al. AJNR Am J Neuroradiol 2009;30:803-807
Medial basal canal
Sphenooccipital synchondrosis
Ecchordosis Physaliphora • Ectopic notochordal
remnant
– Soft tissue mass
associated w
scalloping of
posterior clivus
– T2 bright, non-
enhancing
• Typically incidental:
asymptomatic,
indolent
– Follow-up imaging if
unsure of dx
31 M w HA. Incidental lesion noted that
remained stable in follow-up.
Persistent Canals and Divots Persistent Craniopharyngeal Canal
Type 3A, w cephalocele
Fossa Navicularis Magna
Type 3B, w tumor (teratoma)
CPC: Glastonbury et al, AJNR 2014
Enlarged IT canaliculus Aberrant ICA in ME
Normal FS
Other side
Normal
carotid
canal
Absent FS
Absent foramen spinosum w aberrant ICA, persistent stapedial a
Absent Canals 39 M with R pulsatile tinnitus and vascular retrotympanic mass.
Congenital Absence of ICA/Carotid Canal
Asymmetrical marrow
Petrous Apex Variations: Many Petrous Apex Cephalocele
May coexist with empty sella, arachnoid pitting, osseous-dural dysplasia -- Do consider possible association with IIH (idiopathic intracranial hypertension)
27 M w epilepsy. L temporal origin
of seizures.
Brain may herniate into
arachnoid pits
Patient referred for bx
Dx: prominent arachnoid granulation vs
cephalocele of greater wing of sphenoid
Dx: focal brain herniation into
arachnoid granulation/pit.
Patient did not have IIH.
Another variant to recognize
43 F referred to our sinus center for biopsy of a skull base lesion.
• Benign
developmental
variant
• Nonexpansile
lesion w
osteosclerotic
borders, internal
fat, curvilinear
calcifications in
basisphenoid or
adjacent skull
base
• Fatty marrow
conversion, but
then failure of
pneumatization
Welker et al, AJR June 2008
Dx: arrested pneumatization of the skull base
Focal lesion of R basisphenoid.
Benign or aggressive? HU: -28
DON’T BIOPSY
• These may be actual lesions, but they can
often be identified specifically enough that
they do not require biopsy, or biopsy can be
dangerous, though they may require some
form of treatment
• And in some cases biopsy may be indicated,
but one can still redirect from a plan to
resect to a plan to biopsy
CT and MR are often complementary at
the skull base. We are not just trying to get
the clinicians to order more studies!
56 F w HA, R CN VI palsy
Soft tissue lesion involving sella, central skull base. Met?
Macroadenoma? Should we plan for biopsy?
Let’s do an MR first a CTA/CECT would also be reasonable
Dx: partially thrombosed giant aneurysm of R cavernous ICA.
Patient was subsequently rx’d w flow diverting stents.
21 F w new onset R CN XII palsy and HA during pregnancy.
Dx: presumed “cavernous
hemangioma” [low flow
venous vascular malformation]
of bone. These can enlarge
during pregnancy. Post-
partum, symptoms resolved,
and the patient is being
followed.
ASL perfusion 51 M referred for evaluation of clival chordoma after an MR
was performed for h/o HA
Dx: “benign fibro-osseous lesion generally c/w FD, but islands
of woven bone are rare”
How about this case? 39 M w HA, diplopia.
Dx: metastatic spindle cell sarcoma
48 year old M w 3 year h/o R sided rhinorrhea.
Dx: spontaneous lateral sphenoid cephalocele c/b CSF leak.
Summary
• Anterior
and central
skull base
anatomy
• Anatomical
variants
• Don’t touch
lesions
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